The Third Generation Partnership Project (3GPP) communication standards of Long Term Evolution (LTE) use Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink (DL) and Discrete Fourier Transform (DFT) spread OFDM (DFTS-OFDM) in the uplink (UL). The basic LTE downlink physical resource can thus be seen as a time-frequency grid as shown in FIG. 1, where each resource element (RE) corresponds to one OFDM subcarrier (subcarrier spacing Δf=15 kHz) during one OFDM symbol interval (including cyclic prefix).
The next generation mobile wireless communication system (5G or New Radio [NR]), which is currently under standardization in 3GPP, will also use OFDM in DL and both OFDM and DFTS-OFDM in the UL. In addition to sub-carrier spacing Δf=15 kHz, more subcarrier spacing options will be supported in NR, i.e. Δf=(15×2α) kHz, where α is a non-negative integer.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes (numbered 0-9) of length Tsubframe=1 ms, as illustrated in FIG. 2. Each subframe is further divided into 2 slots each with 7 OFDM symbols in a normal cyclic prefix configuration. A similar frame structure will also be used in NR, in which the subframe length is fixed at 1 ms regardless of the sub-carrier spacing used. The number of slots per subframe depends on the subcarrier spacing configured. The slot duration for (15×2α) kHz subcarrier spacing is given by 2−α ms assuming 14 OFDM symbols per slot.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB) or physical resource blocks (PRB), where a PRB corresponds to one time slot (0.5 ms) of seven symbols (numbered 0-6) in the time domain and 12 contiguous subcarriers in the frequency domain, whereby one PRB consists of 7 by 12 RE. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. The minimum resource unit for scheduling is a PRB pair, i.e. two PRBs over two slots in a subframe. For convenience, PRB is used also to refer to a PRB pair in the rest of the text. In NR, a PRB also includes 12 subcarriers in frequency but may span one or more slots in the time domain. The minimum resource unit for scheduling in NR can be one slot, to achieve reduced latency and increased flexibility. To simplify the discussion, subframe is used when scheduling is discussed. It should, however, be understood that it is also applicable to slot in NR.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe in LTE. A downlink system with 3 OFDM symbols as control is illustrated in FIG. 3. In NR, the exact control signaling is still under discussion, but it is likely that the control signal will also be transmitted in the first OFDM symbols.
Physical Channels and Transmission Modes
In LTE, a number of physical DL channels are supported. A downlink physical channel corresponds to a set of resource elements carrying information originating from higher layers. The following are some of the physical channels supported in LTE:                Physical Downlink Shared Channel, PDSCH        Physical Downlink Control Channel, PDCCH        Enhanced Physical Downlink Control Channel, EPDCCH        
PDSCH is used mainly for carrying user traffic data and higher layer messages. PDSCH is transmitted in a DL subframe outside of the control region as shown in FIG. 3. Both PDCCH and EPDCCH are used to carry Downlink Control Information (DCI) such as PRB allocation, modulation level and coding scheme (MCS), precoder used at the transmitter, etc. PDCCH is transmitted in the first one to four OFDM symbols in a DL subframe, i.e. the control region, while EPDCCH is transmitted in the same region as PDSCH. PDCCH and PDSCH will also be supported in NR.
Similarly, the following physical UL channels are supported in both LTE and NR:                Physical Uplink Shared Channel, PUSCH        Physical Uplink Control Channel, PUCCH        
Different DCI formats are defined in LTE for DL and UL data scheduling. For example, DCI formats 0 and 4 are used for UL data scheduling while DCI formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 2D are used for DL data scheduling. In DL, which DCI format is used for data scheduling is associated with a DL transmission scheme and/or the type of message to be transmitted. The following are some of the transmission schemes supported in LTE:                Single-antenna port        Transmit diversity (TxD)        Open-loop spatial multiplexing        Close-loop spatial multiplexing        Up to 8 layer transmission        
PDCCH is always transmitted with either the single-antenna port or Transmit Diversity scheme while PDSCH can use any one of the transmission schemes. In LTE, a UE is configured with a transmission mode (TM), rather than a transmission scheme. There are 10 TMs, i.e. TM1 to TM10, defined so far for PDSCH in LTE. Each TM defines a primary transmission scheme and a backup transmission scheme. The backup transmission scheme is either single antenna port or TxD. Following is a list of some primary transmission schemes in LTE:                TM1: single antenna port, port 0        TM2: TxD        TM3: open-loop SM        TM4: close-loop SM        TM9: up to 8 layer transmission, port 7-14        TM10: up to 8 layer transmission, port 7-14        
It is noted that in this respect, TM9 and TM 10 are identical, but they differ in other respects. In TM1 to TM6, cell specific reference signal (CRS) is used as the reference signal for both channel state information feedback and for demodulation at a User Equipment (UE), while in TM7 to TM10, UE specific demodulation reference signal (DMRS) is used as the reference signal for demodulation.
Codebook-Based Precoding
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The LTE standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. Currently LTE Advanced supports up to 8-layer spatial multiplexing with up to 32 or 16 transmitter (Tx) antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the transmission structure of precoded spatial multiplexing mode in LTE is provided in FIG. 4.
As seen in FIG. 4, the information carrying symbol vector s is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE) or RE. The number of symbols r is typically adapted to suit the current channel properties.
LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) and hence the received NR×1 (where NR is the number of receive antenna ports at a UE) vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en  (1)where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
SU-MIMO and MU-MIMO
When all the data layers are transmitted to one UE, it is referred to as single user MIMO or SU-MIMO. On the other hand, when the data layers are transmitted to multiple UEs, it is referred to as multi-user MIMO or MU-MIMO. MU-MIMO is possible when, for example, two UEs are located in different areas of a cell such that they can be separated through different precoders (or beamforming) at the Base Transceiver Station (BTS), the two UEs may be served on the same time-frequency resources (i.e. PRBs) by using different precoders or beams. In DMRS based transmission modes TM9 and TM10, different DMRS ports and/or the same DMRS port with different scrambling codes can be assigned to the different UEs for MU-MIMO transmission. In this case, MU-MIMO is transparent to UE, i.e., a UE is not informed about the co-scheduling of another UE in the same PRBs.
MU-MIMO requires more accurate downlink channel information than in SU-MIMO in order for the eNB to use precoding to separated the UEs, i.e. reducing cross interference to the coscheduled UEs.
Channel State Information Reference Signal (CSI-RS)
In LTE Release 10 (Rel-10), a new channel state information reference signal (CSI-RS) was introduced for the intent to estimate channel state information. The CSI-RS based CSI feedback provides several advantages over the CRS based CSI feedback used in previous releases. Firstly, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density (i.e., the overhead of the CSI-RS is substantially less). Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements (e.g., which CSI-RS resource to measure on can be configured in a UE specific manner).
Two types of CSI-RS are defined in LTE: non-zero power (NZP) and zero power (ZP) CSI-RS. NZP CSI-RS can be used to estimate the effective channel of a serving transmission point (TP), while ZP CSI-RS can be used to measure interference, or to prevent interference to other UEs receiving signals in the ZP CSI-RS resource elements. For simplicity, NZP CSI-RS may occasionally be referred to as CSI-RS in this disclosure.
By measuring on a CSI-RS, a UE can estimate the effective channel the CSI-RS is traversing, including the radio propagation channel and antenna gains. In more mathematical rigor this implies that if a known CSI-RS signal x is transmitted, a UE can estimate the coupling between the transmitted signal and the received signal (i.e., the effective channel). Hence if no virtualization is performed in the transmission, the received signal y can be expressed asy=Hx+e  (2)and the UE can estimate the effective channel H.
Up to eight CSI-RS ports can be configured for a Rel-11 UE, that is, the UE can thus estimate the channel from up to eight transmit antennas. In Rel-13, up to 16 CSI-RS ports are supported.
FIG. 5 shows the REs available for CSI-RS allocations in a PRB. Up to 40 REs can be configured for CSI-RS. CSI-RS is transmitted over all PRBs of the downlink system bandwidth in order for a UE to measure CSI over the whole bandwidth.
CSI-RS can be transmitted periodically on certain subframes, also referred as CSI-RS subframes. A CSI-RS subframe configuration consists of a subframe periodicity and a subframe offset. The periodicity is configurable at 5, 10, 20, 40 and 80 ms.
A CSI-RS configuration consists of a CSI-RS resource configuration and a CSI-RS subframe configuration.
Codebook Based Channel State Information (CSI) Estimation and Feedback
In closed loop MIMO transmission schemes such as TM9 and TM10, a UE estimates and feeds back the downlink CSI to the evolved Node B (eNB). The eNB uses the feedback CSI to transmit downlink data to the UE. The CSI consists of a transmission rank indicator (RI), a precoding matrix indicator (PMI) and a channel quality indicator(s) (CQI). A codebook of precoding matrices for each rank is used by the UE to find out the best match between the estimated downlink channel Hn and a precoding matrix in the codebook based on certain criteria, for example, the UE throughput. The channel Hn is estimated based on a NZP CSI-RS transmitted in the downlink for TM9 and TM10.
The CQI/RI/PMI together provide the downlink channel state of a UE. This is also referred to as implicit CSI feedback since the estimation of Hn is not fed back directly. The CQI/RI/PMI can be wideband or sub-band depending on which reporting mode is configured.
The RI corresponds to a recommended number of data symbols/streams that are to be spatially multiplexed and thus transmitted in parallel over the downlink channel. The PMI identifies a recommended precoding matrix codeword (in a codebook which contains precoders with the same number of rows as the number of CSI-RS ports) for the transmission, which relates to the spatial characteristics of the channel. The CQI represents a recommended transport block size (i.e., code rate) and LTE supports transmission of one or two simultaneous (on different layers) transmissions of transport blocks (i.e. separately encoded blocks of information) to a UE in a subframe. There is thus a relation between a CQI and an Signal-to-Interference-plus-Noise Ratio (SINR) of the spatial stream(s) over which the transport block or blocks are transmitted.
A codebook of up to 32 (previously 16) antenna ports has been defined in LTE. Both one-dimensional (1D) and two-dimensional (2D) antenna array are supported. For LTE Rel-12 UE and earlier, only a codebook feedback for a 1D port layout is supported, with 2, 4 or 8 antenna ports. Hence, the codebook is designed assuming these ports are arranged on a straight line. In LTE Rel-13, codebooks for 2D port layouts were specified for the case of 8, 12 or 16 antenna ports. In addition, a codebook 1D port layout for the case of 16 antenna ports was also specified in LTE Rel-13.
In LTE Rel-13, two types of CSI reporting were introduced, i.e. Class A and Class B. In Class A CSI reporting, a UE measures and reports CSI based on a codebook for the configured 2D antenna array with 8, 12 or 16 antenna ports. The Class A codebook is defined by five parameters, i.e. (N1, N2, O1, O2, Config), where (N1, N2) are the number of antenna ports in a first and a second dimension, respectively and (O1,O2) are the DFT oversampling factors for the first and the second dimension, respectively. Config ranges from 1 to 4 and defines four different ways the codebook is formed. For Config=1, a PMI corresponding to a single 2D beam is fed back for the whole system bandwidth while for Config∈{2, 3, 4}, PMIs corresponding to four 2D beams are fed back and each subband may be associated with a different 2D beam. The CSI consists of a RI, a PMI and a CQI or CQIs, similar to the CSI reporting in pre Rel-13.
In Class B CSI reporting, in one scenario (also referred to as “K>1”), the eNB may pre-form multiple beams in one antenna dimension. There can be multiple ports (1, 2, 4 or 8 ports) within each beam on the other antenna dimension. “Beamformed” CSI-RS are transmitted along each beam. A UE first selects the best beam from a group of beams configured and then measures CSI within the selected beam based on the legacy codebook for 2, 4 or 8 ports. The UE then reports back the selected beam index and the CSI corresponding to the selected beam. In another scenario (also referred to as “K=1”), the eNB may form up to 4 (2D) beams on each polarization and “beamformed” CSI-RS is transmitted along each beam. A UE measures CSI on the “beamformed” CSI-RS and feedback CSI based on a new Class B codebook for 2, 4, 8 ports.
In LTE Rel-14, Class-A codebooks for additional one- and two-dimensional port layouts with 8, 12, 16, 20, 24, 28 and 32 antenna ports were specified. In addition, an advanced Class-A codebook was introduced with higher resolution channel feedback to support MU-MIMO operations. However, a UE can only be configured semi-statically with either the regular Class-A codebook based feedback or the advanced codebook-based CSI feedback.
CSI Process
In LTE Release 11, CSI processes are defined such that each CSI process is associated with a CSI-RS resource and a CSI-IM resource. A CSI-IM resource is defined by a ZP CSI-RS resource and a ZP CSI-RS subframe configuration. A UE in transmission mode 10 can be configured with one or more (up to four) CSI processes per serving cell by higher layers and each CSI reported by the UE corresponds to a CSI process. The multiple CSI processes were introduced to support Coordinated Multi-Point (COMP) transmission in which a UE measures and feeds back CSI associated with each transmission point to an eNB. Based on the received CSIs, the eNB may decide to transmit data to the UE from one of the TPs.
CSI Reporting
For CSI reporting, both periodic and aperiodic (i.e. triggered by eNB) reports are supported, known as P-CSI and A-CSI respectively. In a CSI process, a set of CSI-RS ports are configured for which the UE performs measurements. These CSI-RS ports can be configured to be periodically transmitted with 5 ms, 10 ms, 20 ms etc. periodicity. The periodic report may use PUCCH format 2, or its variants (2a, 2b) and has a configured periodicity as well, e.g. 20 ms. RI, PMI, CQI may be reported on different subframes in some case due to the payload size limitation of PUCCH format 2.
In case of aperiodic CSI reporting, a UE reports CSI only when it is requested by the eNB. A CSI reporting request is carried in an uplink DCI (i.e. DCI 0 or DCI 4) and the corresponding report is carried in a PUSCH configured by the DCI. PUSCH is generally able to carry a much larger payload than PUCCH and thus CSI can be sent in one subframe.
For a given CSI process configured for a UE, both periodic and aperiodic CSI reporting can be configured. Periodic CSI can be used for the UE to report CSI periodically even there is no data to send to the UE. This can be used to obtain long term CSI at the eNB. On the other hand, aperiodic CSI can be used only when there is data to transmit to the UE, it can provide more instantaneous CSI to track fast channel variations and thus better channel utilization.
Different feedback reporting modes may be used for periodic and aperiodic CSI reporting. For example, wideband PMI and CQI report could be configured for periodic CSI reporting while subband PMI and CQI report could be configured for aperiodic CSI reporting. However, for the same CSI process, the same codebook is used for both.
In some scenarios, the CSI feedback may be restricted to a subset of the codewords in a codebook by means of codebook set restriction configuration. In this case, a UE measures CSI based on the configured subset of codeword in the codebook. In some other cases, subsampling of the codebook can be used to reduce the periodic CSI feedback overhead.
When multiple CSI processes or multiple downlink carriers (or cells) are configured for a UE, CSI reporting configuration can be different for different cells or different CSI processes. However, for a NZP CSI-RS configuration in a CSI process, only one codebook per rank can be configured for CSI measurement and reporting; different codebooks for the same CSI-RS configuration is not supported in LTE.